1. Field of the Invention
The present invention relates to diaphragm-type carburetors preferably for use in general purpose engines and provided with a metering chamber maintaining a desired fuel pressure by a diaphragm and a lever mechanism, and, more particularly, to diaphragm-type carburetors provided with a means for removing a bubble of a fuel vapor introduced to the metering chamber or generated within the metering chamber.
2. Description of Related Art
A diaphragm-type carburetor frequently is used for supplying a fuel to a general purpose engine, particularly, a general purpose engine having a small displacement, and an inner portion of such diaphragm-type carburetors generally is provided with a metering chamber (e.g., a constant pressure chamber) for obtaining a constant fuel pressure which provides a reference for fuel metering by a metering diaphragm and a metering lever mechanism.
Such a metering chamber may be filled only with a liquid fuel to achieve a stability of a pressure. Nevertheless, a fluctuation in a fuel discharge pressure by a fuel pump may be generated due to various factors such as when the temperature of a portion around the fuel pump for delivering the fuel to the metering chamber becomes elevated due to the heat of the engine, the pressure of the fuel increases, the fuel then is fed into a discharger chamber, and the pressure of the fuel decreases, so that fuel vapor is generated within the metering chamber. In particular, a fuel vapor bubble may be generated due to elevated environmental temperatures during summer time so as to create a problem.
If the bubble of the generated fuel vapor is introduced to the metering chamber, and is delivered to an intake passage from a main nozzle or the like, a reduction of engine speed and an engine stoppage may occur due to a lean air-fuel mixture. In Japanese Unexamined Patent Publication No. H09-158806, a technique for suppressing a high negative pressure within a pulse chamber is described in a no load or low load operation region and eliminating an excessive increase of a fuel discharge pressure by connecting an upstream portion of a throttle valve of a carburetor intake passage to a pulse chamber of a fuel pump in a structure which uses a pulsation pressure of an intake pipe as a driving force of the fuel pump.
Further, in Japanese Unexamined Patent Publication No. 2000-297702, a means for discharging a fuel vapor stored in an upper portion of a metering chamber of a carburetor is described. The means for discharging fuel vapor is a purge pump which uses a pulsation pressure of an intake pipe as a driving force. In Japanese Unexamined Patent Publication No. 2003-166444, structure is described for withdrawing and discharging a fuel vapor to an intake passage by opening one end of a bubble evacuating passage to a downstream portion of a fuel inflow control valve of a fuel passage flowing into a metering chamber, opening the other end to a midsection of the fuel passage connected to a main nozzle from the metering chamber, and utilizing a specific gravity difference between the fuel vapor and the fuel to remove the fuel vapor.
Nevertheless, the technique described in Japanese Unexamined Patent Publication No. H09-158806 may not be applied to a structure using the pulsation pressure of the crank chamber as the driving force of the fuel pump. Further, the technique described in Japanese Unexamined Patent Publication No. 2000-297702 and the technique described in Japanese Unexamined Patent Publication No. 2003-166444 may not completely eliminate the problem that the fuel vapor is discharged to the intake passage from the upper portion of the metering chamber so as to make the air-fuel mixture excessively lean, and create a risk that the fuel pressure within the metering chamber becomes unstable.